TheChowsterwrote:
1. When there are so many unanswered questions that you have at the end. Which gives you this incomplete feeling of the anime

Depends on the anime. I think Darker than Black was really good, even if the ending raised a ridiculous number of questions than answered at the end. Twice.
Also, NOOOOOOOO Hobo Chinese Electric Batman!!!!!!!!

The worst way to end an anime is on a 'read the manga *troll face*' ending. An anime should be its own thing and not rely on you having to go to the manga or whatever the source material is to finish it. I'm all right with a show if it finishes in a way where there could be more but everything is still neatly wrapped up, it's when it doesn't finish properly it annoys me:

As my most recent example, I watched Umineko no Naku Koro Ni, which ends on a cliffhanger and apparently only covers 4 of the 8 novels. It's been 3 years since the anime was released, so the chances of there being a second season seem rather slim, and it pissed me off to know end to know I'd only seen half the story and would have to buy into a completely different medium to see it finished. I'm only all right with it not finishing if they can confirm that there is already a second season in the works that isn't going to solely rely on funds made from the first season. Now I have to go on knowing that I'll never know how that anime finished. It's a shoddy marketing scam if you ask me. To me, the ending is paramount, and if you can't end it properly I will be incredibly annoyed.

That's my main peeve. Some smaller ones would be:

Rushed endings. It especially stands out when a show has had reasonable pacing throughout, and then a shit ton of stuff happens in the last episode to wrap everything up. It feels cheap and silly as if the creator just couldn't be arsed in the end and was in a rush to get everything over with.

I also hate when a romance anime ends without the characters even kissing. I make an exception with Clannad but it always annoys me when that happens. Also on the topic of romance, one of my big problems with harem anime is when the MC doesn't end up with any of the characters. Then it all feels like whatever romantic encounters happened were completely pointless, and that you just wasted your time.

The everyone dies ending. That often leaves you feeling depressed, and it can even be used as a cheap cop-out in an effort to seem deep. It can be done well, but not very often. In fact, I don't recall ever seeing a show where everyone dies and thinking to myself 'that's the best way it could've ended'.

And, as some have pointed out, the 'it was all a dream'. Yeah, that show you just spent however many hours watching/ manga you read? None of it even happened lulz. Nothing feels like a bigger waste of time, than watching something that never even happened since it makes you realise that none of what happens mattered to any of the characters given how it was all in one person's mind. Obviously when you watch something/ read something fictional it doesn't matter since you know it didn't happen, but watching how it affects the characters is one of the best bits in my opinion. Knowing that even they weren't affected by anything you just saw makes it seem meaningless, since nothing that happened even mattered to the characters, let alone you. I tie it into my thoughts of 'almost anyone can dream of really insane shit happening' and as such it makes what you just saw seem that less special. That it didn't happen in their reality, that it was a dream, just like any of us could have.

I'm not against this on principle only because I saw it used brilliantly well once as a plot device.

Where?

It wasn't an anime, to be fair; it was a movie

Spoiler Alert! Click to show or hide

from 2005 with Jason Statham called Revolver, directed by Guy Ritchie.

More info:

Spoiler Alert! Click to show or hide

Statham plays a professional gambler dash con artist who's been in jail for seven years, and when he gets out he goes for revenge against the guy who backstabbed him who also runs a casino.

And the finale spoiler:

Spoiler Alert! Click to show or hide

The twist plays in that the entire movie is a dream the night before Statham's release, all the events and probabilities played out in his mind. So it was effectively like time travel, he saw what could happen, and he saw that his revenge was ultimately pointless and empty, and at the end of the film he has the chance to decide between pursuing his plan knowing that it won't get him what he wants or forging a new path.

I'm not against this on principle only because I saw it used brilliantly well once as a plot device.

Where?

It wasn't an anime, to be fair; it was a movie

Spoiler Alert! Click to show or hide

from 2005 with Jason Statham called Revolver, directed by Guy Ritchie.

More info:

Spoiler Alert! Click to show or hide

Statham plays a professional gambler dash con artist who's been in jail for seven years, and when he gets out he goes for revenge against the guy who backstabbed him who also runs a casino.

And the finale spoiler:

Spoiler Alert! Click to show or hide

The twist plays in that the entire movie is a dream the night before Statham's release, all the events and probabilities played out in his mind. So it was effectively like time travel, he saw what could happen, and he saw that his revenge was ultimately pointless and empty, and at the end of the film he has the chance to decide between pursuing his plan knowing that it won't get him what he wants or forging a new path.

I find that 'power of friendship' thing to be sort of the measure of a writer. There are some writers who get in over their heads, in order to try to make things seem more dramatic, they give the enemy an undeniable advantage. However sometimes they go so far with this, that they themselves can't figure out a plausible way for the hero to win, so they end up relying on deus ex machina.

I like writers best who can avoid using that and write a well foreshadowed, clever resolution. Unfortunately, off the top of my head I can't think of any Shonen manga / anime that hasn't relied on a convenient new power out of now where to fix a problem. Yes, even Togashi of Hunter x Hunter falls into this trap.